1. Technical Field
This invention relates to zoom lens systems, and more particularly, to such lens systems which are capable of operating in the infrared spectrum.
2. Background of the Invention
There are many applications for optical lens systems capable of operating in the infrared spectrum. For purposes of this invention, the term "infrared" means electromagnetic radiation having a wavelength longer than visible radiation and shorter than microwave radiation. The numerical wavelength range of the infrared spectrum is usually considered to extend from 0.7 microns which is the longest visible wavelength to substantially 100 microns. Infrared lens systems can be used as part of a night viewing surveillance device, an aircraft navigation device used to provide all weather and night flying capabilities, as well as known infrared radar and imaging systems. One such system is generally referred by the acronym "FLIR" derived from the words "forward looking infrared". These systems are preferably operated in the 2-20 micron wavelength region of the spectrum, more particularly in the 8-12 micron range.
It has been conventional practice to mount infrared optical sensors within a pod or turret mounted on the aircraft. Servomechanisms are used to rotate or otherwise articulate the pod to permit the sensor's field of view to be oriented in a variety of directions. For protection from the airstream, this sensor is usually mounted behind a protective window. The use of a flat window unfortunately creates a significant aerodynamic drag on the pod which can make movement of the pod difficult at high speeds. Accordingly, the use of a curved rather than a flat window in aircraft applications is desirable since a reduction in the aerodynamic drag would not only reduce the torque needed to be produced by the pod servomechanisms, but would also reduce the amount of fuel consumed by the aircraft as well as reduce mechanical vibrations on the pod or turret.
Even further reductions in aerodynamic drag can be produced if the curved window consists of a shell or dome of material having a band pass in the infrared spectrum, such as zinc selenide, zinc sulfide or germanium. Unfortunately, these domes generally possess weak negative optical power, and create spherical aberrations, and chromatic aberrations. In addition, if the optical sensor is positioned along a line which does not pass through the center of the dome, i.e., if the dome is decentered, additional aberrations such as axial coma are also introduced.
A variety of different attempts have been made to permit the use of this so-called "decentered" dome approach which is generally recognized as being necessary in supersonic aircraft applications where aerodynamic drag must be kept to an absolute minimum. Unfortunately, many of the prior approaches to correct these problems were relatively costly, often requiring additional lenses resulting in undesirable transmission losses, or provided generally unacceptable performance characteristics. In addition, these aberration compensation techniques were generally not capable of compensating for different amounts of dome decentration.
It is often also desirable to provide a zoom lens system that is capable of operating in the infrared spectrum which can be used as a "bolt on" attachment to a primary imaging system such as that encountered in conventional FLIR systems. Typically, the zoom system must be of the afocal type wherein the output of the zoom is a collimated beam of energy focused at infinity. Among the desirable design parameters is that the afocal infrared zoom provides small transmission losses while at the same time providing usefully high image quality. As noted above, transmission losses are particularly acute with lenses having a spectral bandpass in the infrared region. Consequently, both from a cost and performance standpoint the number of lenses must be kept to a minimum. In addition, a compact design is also highly desirable.
In addition to the design objectives noted above, there exists a need for an infrared zoom lens system capable of providing continuous-in focus field of view of a scene (i.e., afocal type), with the zoom lens being capable of a relatively large magnification ratio while providing the capability of having a unit magnification power (1.0.times.) mode of operation and at the same time providing a compact structure. The capability to provide a unit power zoom position is important for navigation applications which require the displayed scene to correspond exactly to the actual scene the pilot would see with direct viewing. The unit power zoom position would provide the pilot with the exact field of view necessary to fly his aircraft.
Those skilled in the art can thus appreciate that a variety of advantages could be obtained if it were possible to provide a zoom lens telescope system that can be used with a decentered dome. For example, low aerodynamic drag can be achieved while at the same time providing the primary imaging system with the advantages of a zoom lens attachment.